1. Field of the Invention
The present invention relates generally to a pulse waveform generator circuit, and more particularly to a pulse wave-form generator circuit for use as a tape head switching device in a video tape recorder and which can be easily made as an integrated circuit.
2. Description of the Prior Art
In an apparatus which can record a video signal on a magnetic tape and then reproduce a recorded video signal from the magnetic tape, as shown in FIG. 1A, there is employed a rotary magnetic head assembly in which two rotary magnetic heads 2 and 3 are attached to a rotary body 1 with an angular distance of 180.degree.. A magnetic tape 4 travels around the rotary body 1 with an angular range of about 180.degree. in an oblique direction with respect to the rotary surface of the rotary magnetic heads 2 and 3.
With this kind of apparatus, it is required that output signals from the two rotary magnetic heads 2 and 3 must be switched over one to the other in order to create a continuous output signal which has no overlapping portion. To achieve this purpose, two pulse producers 5 and 6 are provided for detecting the pass by of the rotary magnetic heads 2 and 3, as shown in FIG. 1B. The pulse signals obtained from the pulse producers 5 and 6 are used to trigger switching pulse signals for controlling the output signals from the two rotary magnetic heads 2 and 3 and also for coordinating the head positions with respect to the frames or fields of the video signal.
The present state of the art method for generating a video tape head switching signal will be described with reference to FIGs. 2A to 2G.
A pulse signal S.sub.A derived from one of the pulse producers 5 or 6 shown in FIG. 2A is applied to a mono-stable multivibrator (not shown) which may produce a pulse signal S.sub.C with a predetermined pulse width as shown in FIG. 2C. A pulse signal S.sub.B shown in FIG. 2B is obtained from the other pulse producer and is applied to another mono-stable multivibrator (not shown) which may produce a pulse signal S.sub.D with a predetermined pulse width as shown in FIG. 2D. The reason for the predetermined pulse widths of signals S.sub.C and S.sub.D is that the positioning of the pulse producers 5 and 6 is carried out electrically. The pulse producers 5 and 6 are located in such manner a manner the pulse signals S.sub.A and S.sub.B are derived shortly before the time when the video output signals from the magnetic heads 2 and 3 are to be switched, and the time constants of the above two mono-stable multivibrators, that is the pulse widths of the pulse signals S.sub.C and S.sub.D, are suitably selected such that the trailing edges of the pulse signals S.sub.C and S.sub.D coincide with times when the video output signals from the heads 2 and 3 are to be switched. As a result, there is no need to locate the pulse producers 5 and 6 at precise, predetermined positions, and hence, rough positioning of the units is permissible.
A desired waveform for switching the video output signals from the heads 2 and 3 is shown in FIG. 2G by S.sub.H. This pulse signal S.sub.H is reversed sequentially at the trailing edges of the pulse signals S.sub.C and S.sub.D. In order to obtain a desired pulse signal S.sub.H, the pulse signal S.sub.C is usually applied in the prior art to a differentiation circuit (not shown) to produce differentiated pulses P.sub.1 and P.sub.2, shown in FIG. 2E, while the other pulse signal S.sub.D is applied to another differentiation circuit (not shown) to produce differentiated pulses P.sub.3 and P.sub.4 shown in FIG. 2F. The pulse signals P.sub.2 and P.sub.4 obtained at the trailing edges of the pulse signals S.sub.C and S.sub.D are applied to a flip-flop circuit (not shown) to reverse the same sequentially and to produce the switching pulse signal or waveform S.sub.H.
However, the disadvantage associated with the above prior art device is the difficulty in converting to an integrated circuit. The differentiation circuits above made use of capacitors, and as the number of parts to be connected externally to an integrated circuit increases the number of external terminals undesirably increases.